Palaeozoic Era

The Palaeozoic Era extends from 538.8 to 251.902 million years ago. It is the first era of the Phanerozoic Eon and contains six periods: Cambrian, Ordovician, Silurian, Devonian, Carboniferous and Permian. Across this interval, diverse marine animal communities became established, complex ecosystems spread onto land, vertebrates acquired limbs and amniotes became independent of open water for reproduction.

No non-avian dinosaur lived in the Palaeozoic. Dinosaurs appeared after the era had ended, during the Late Triassic. Sail-backed Dimetrodon, giant arthropods and many early tetrapods are often grouped with dinosaurs in popular images, but they belonged to different lineages and much older ecosystems.

Cambrian seas and the expanding animal record

Early Cambrian rocks contain abundant trilobites, brachiopods, sponges, molluscs, echinoderms and early chordates. Exceptional deposits also preserve soft-bodied radiodonts, lobopodians and delicate organs that ordinary fossilisation removes. These sites show why hard-part fossils alone understate the diversity and anatomy of ancient communities.

The increase in visible fossils involved several connected changes: more mineralised skeletons, deeper and more complicated burrowing, new predatory interactions and better preservation in some settings. Evolutionary origins began before many groups became abundant enough to leave an obvious record.

Ordovician diversification and the first major crisis

During the Ordovician, diversity increased within many established marine groups. Planktonic food webs, reef communities and animals living on and within the seafloor expanded. The earliest convincing spores from land plants are small, but they indicate that continental ecosystems were beginning to change.

The end-Ordovician extinction occurred in pulses associated with rapid glaciation, falling sea level and later environmental reversal. Because most known biodiversity lived in shallow seas, loss of continental shelf habitat had wide consequences. Recovery unfolded within new oceanic and climatic conditions.

Silurian shores and vascular plants

Silurian rocks preserve small vascular plants with conducting tissues and branching axes. Arthropods also occupied terrestrial settings. These pioneers did not form modern forests, but they helped stabilise sediment, develop soils and create food and shelter away from the water.

In the sea, jawed fishes diversified alongside jawless groups. Reefs and shelf communities recovered after the Ordovician crisis. The distinction between a period boundary and a sudden worldwide ecological switch remains important: biological changes crossed boundaries at different rates in different regions.

Devonian forests, fishes and early tetrapods

The Devonian saw the spread of trees and deeper root systems. Roots altered erosion and river channels, while plant growth and burial affected carbon cycling. By the Late Devonian, forests had created vertically structured habitats and changed the delivery of nutrients to rivers and seas.

Lobe-finned fishes included lineages close to tetrapods. Fossils such as Tiktaalik, Acanthostega and Ichthyostega document combinations of fish-like and tetrapod features, but they do not form a simple ladder. Late Devonian extinctions were a series of disturbances rather than one sharply instantaneous event.

Carboniferous wetlands and the amniotic egg

Large equatorial wetlands accumulated plant material that later became coal. Oxygen levels were high during parts of the Carboniferous and early Permian, which may have helped some arthropods attain large sizes. Climate, ecology and lineage history also mattered, and most arthropods were not gigantic.

Early amniotes evolved membranes that allowed embryos to develop away from open water. The lineage later divided into synapsids, leading towards mammals, and sauropsids, including reptiles and birds. This split long predates dinosaurs and explains why Dimetrodon is closer to mammals than to any dinosaur.

Permian Pangaea and terrestrial specialisation

By the Permian, continental collision had assembled Pangaea. Its large interior promoted strong seasonality and extensive dry environments, although coastlines and high latitudes retained other habitats. Seed plants became increasingly important. Synapsids ranged from herbivores to large predators, while reptiles diversified in parallel.

The fossil record does not depict one “Permian fauna”. Early pelycosaur-grade synapsids, later therapsids and regional communities were separated by tens of millions of years. Exact age, formation and locality are needed before two animals can be treated as contemporaries.

The end-Permian mass extinction

The Palaeozoic ended with the most severe recognised mass extinction of the Phanerozoic. Vast Siberian Traps eruptions released gases that drove rapid warming and disturbed carbon cycling. Ocean deoxygenation, acidification and lethal temperature stress affected marine systems, while terrestrial ecosystems also suffered major turnover. Its place among the Big Five mass extinctions becomes clearer when the events are compared with the same evidence rules.

The boundary at 251.902 Ma begins the Triassic and Mesozoic Era. Survival did not restore the previous world. Low-diversity communities, repeated environmental stress and slow rebuilding characterised much of the earliest Triassic.

How confidently can the era be reconstructed?

Marine shells are abundant and useful for correlation, but land records are patchier. Plants may be represented by spores, roots, wood or compressions that sample different parts of an organism. Trackways record movement without necessarily identifying the trackmaker. Vertebrate skeletons can mix bones transported from different habitats.

Reliable reconstructions combine anatomy with sedimentology, geochemistry, precise stratigraphic position and regional comparison. The general sequence is strongly established; exact atmospheric composition, local climate and the timing of biological responses retain wider uncertainty. Use the time calculator to place any numerical Palaeozoic age in the correct period.